Breast cancer is the second leading cause of cancer-related death among women in the United States. Despite efforts to increase awareness and develop novel treatments, there is much to learn about the molecular etiology of breast cancer. Therefore, it is important to characterize novel breast-cancer related proteins that may be used for diagnosis, prognosis, and as markers for the best course of treatment. The human DEK gene was first identified as a fusion with the CAN/NUP214 nucleoporin gene in AML patients. It is ubiquitously expressed, chromatin-binding protein in proliferating cells with potential roles in transcriptional regulation, DNA topology, replication, and repair. As a proto-oncogene, DEK inhibits cellular senescence, differentiation, and apoptosis in primary keratinocytes and in cervical cancer cells, presumably through regulation of the p53 protein family. However, its oncogenic activities in vivo remain poorly understood. Gene amplification and upregulated mRNA expression have been described for the human DEK gene in hematopoietic cancers and solid tumors. Meta-analysis using Oncomine and my preliminary data indicated that it also may be important for breast cancer pathogenesis: DEK mRNA and protein overexpression was observed in primary and cultured breast cancers compared to normal mammary tissue, and overexpression strongly correlated with several clinico-pathological variables. DEK protein expression and potential oncogenic activity have not been studied in most cancers, including breast cancer. My preliminary data suggest that DEK may influence DNA repair via the repression of ATM. Based on our data, I hypothesize that DEK is involved in breast cancer development through interference with cell cycle checkpoints and DNA damage response pathways. Three specific aims are proposed to test this hypothesis: (1) Determine whether DEK expression is elevated in murine and human primary breast cancer specimens and cultured cell lines;(2) Determine if DEK upregulation stimulates tumor phenotypes in cell culture and xenograft models of breast cancer;and (3) Delineate the role of DEK in breast cancer development in vivo. PUBLIC HEALTH RELEVANCE: These experiments will determine the role of DEK in the initiation and progression of breast cancer, and will generate a murine model system to support future preclinical studies. Recent data from our laboratory support the targeting of DEK as a potential clinical approach with the finding that the rate of cellular proliferation directly correlates with the degree of cell death following DEK depletion: differentiated cells are not susceptible, whereas rapidly proliferating tumor cells undergo dramatic cell death. We hypothesize that the targeting of DEK in breast cancer may be of therapeutic value.